Fine structural studies on nerve and muscle cells will be continued, with ancillary observations on other cell types. Work on long-sarcomere annelid muscles containing large-diameter paramyosin filaments will be extended by further observations on these and their relationship with the actin filaments. Freeze-fracture studies on the frog sartorius muscle (normal and denervated) are continuing and will involve comparative observations on other striated muscles. Work on the proposed role of axoplasmic microtubules in translocation of particulates will proceed with a statistical analysis of the distribution of mitochondria and microtubules in axons of various sizes in the lamprey central nervous system, since recent electron micrographs suggest a spatial association between these components. High resolution study will be made of membrane-limited aggregates of microtubules (in a fish venom gland) that appear to be resistant to depolymerization by vinblastin.